1. Field of the Invention
The present invention relates generally to a method of fabricating automotive drive shafts and more specifically, to a method of attaching metal end-fittings such as an automotive U-joint yoke to tubular shafts which rotate at high rates of speed and transmit torque and axial forces such as when in use as a vehicle drive shaft.
2. Prior Art
The attachment of end-fittings to metal tubes has historically been accomplished by welding the joint between the end-fittings and the tubes. Many other techniques are available with varying degrees of success. Among these other methods are the use of pins, rivets, bolts, adhesives, and such mechanical methods as splines, keyways, polygon matching shapes, shrink fits and press-fits. However, none of these attachment methods has been satisfactorily applied to drive shafts of vehicles or are in continuing use in vehicles today. One recent innovation used in the process of attaching an end-fitting to a tubular drive shaft, known under the trademark MAGNAFORM, employs a very high electromagnetic-induced force to swage a tube onto a fitting and is in use today, but with limited success. In one such electromagnetic force-induced attachment process being used today by a major automotive company, a yoke-type end-fitting seen in FIG. 1 of the accompanying drawings is provided with a plurality of axially directed splines of substantially rectangular or trapezoidal cross-section and a circumferentially disposed recess. The outer diameter of the splines is sufficiently small to allow the splines to be placed into the end of the tubular drive shaft. A sleeve is then placed on the outside of the tubular drive shaft in substantial alignment with the splines. Prior to the application of the aforementioned electromagnetic swaging force, the outer sleeve is sufficiently larger in diameter than the tubular drive shaft to allow it to be easily slid over the exterior surface of the drive shaft. The combination of drive shaft, end-fitting and sleeve is then subjected to the aforementioned electromagnetic forces to swage the sleeve thereby reducing its diameter until it firmly grasps the drive shaft tube and the splines of the yoke end-fitting. In fact, the force is so great that it causes the sleeve and the tube to become deformed and take on a rough replication of the shape of the splines and the circumferential recess, thereby firmly attaching the drive shaft tube to the yoke end-fitting. The circumferential recess is designed to prevent axially directed forces from permitting the end-fitting to slip out of the tube and the splines are primarily intended to prevent slippage between the tube and the end-fitting upon the application of a high level of torque through the drive shaft. Unfortunately, the results of such a method for attaching the drive shaft tube to the end-fitting have been less than satisfactory. More specifically, it has been found that irrespective of the force used to electromagnetically swage the outside sleeve to the tube, after the electromagnetic force is removed there is an inherent relaxation of the metal of which the sleeve and the tube are comprised. Consequently, when torque is applied to the drive shaft in the vehicle there is a small amount of slippage between the splines and the drive shaft tube which produces a loud and irritating sound which has resulted in a large number of consumer complaints.
A large number of revisions have been made in order to attempt to solve this problem. By way of example, the number of splines has been increased and decreased. The shape of the splines has been changed. The materials have been coated with a variety of additional materials including nickel, copper, anodizing and abrasive paints. Wall dimensions have been varied. The splines have been sandblasted. Various lubrications have been applied to the splines and the interior of the drive shaft tube. The application of electromagnetic force to swage the exterior sleeve has been varied including applying the force twice to increase the forces associated with the attachment process. O-rings have been placed between the splines and the interior of the drive shaft tube. The yoke has been tapered, riveted and pinned and the round shape of the tube has been changed to oval. Unfortunately, all of these attempts at solving the aforementioned problem have been unsatisfactory and the main disadvantage previously described, namely, a loud, unpleasant sound of slippage upon the application of torque between the splines and the drive shaft tube, has persisted. There is therefore a need to provide a solution which permits the advantageous use of swaging a tube onto the drive shaft fitting while concurrently solving the aforementioned problem to provide a repeatedly reliable end-fitting attachment which does not produce any slippage, particularly any slippage discernable in the unpleasant audible manner as previously described.